Copolymer for Solar Cell Applications
GUAN-YU CHEN,1SHANG-CHE LAN,1PO-YU LIN,1CHIH-WEI CHU,2,3KUNG-HWA WEI1 1
Department of Material Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan 2
Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan 3
Department of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan
Received 26 May 2010; accepted 9 July 2010 DOI: 10.1002/pola.24235
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT:In this study, we synthesized a new polymer, PCTDBI, containing alternating carbazole and thiadiazole-ben-zoimidazole (TDBI) units. This polymer (number-average mo-lecular weight ¼ 25,600 g mol1), which features a planar imidazole structure into the polymeric main chain, possesses reasonably good thermal properties (Tg¼ 105C;Td¼ 396C) and an optical band gap of 1.75 eV that matches the maximum photon flux of sunlight. Electrochemical measurements revealed an appropriate energy band offset between the poly-mer’s lowest unoccupied molecular orbital and that of PCBM,
thereby allowing efficient electron transfer between the two species. A solar cell device incorporatingPCTDBI and PCBM at a blend ratio of 1:2 (w/w) exhibited a power conversion effi-ciency of 1.20%; the corresponding device incorporating PCTDBI and PC71BM (1:2, w/w) exhibited a PCE of 1.84%. VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4456–4464, 2010
KEYWORDS:carbazole; conjugated polymer; copolymerization; heteroatom-containing polymers; imidazole; solar cell
INTRODUCTION Research into conjugated polymers possess-ing extended arrays of delocalizedp-electrons has advanced dramatically because of their potential application in organic optoelectronic devices, especially for the development of or-ganic solar cells based on bulk heterojunctions (BHJs) of conjugated polymers.1–5 Solution-processed BHJ solar cells are simple-to-prepare devices featuring an electron-donating conjugated polymer and an electron-accepting fullerene as the active layer. Significant progress has been achieved recently on improving the power conversion efficiency (PCE) of polymer BHJ solar cells. For example, BHJ polymer solar cells based on regioregular poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester (rr-P3HT/PC61BM)
composites can exhibit PCEs of up to 6%.6–10To increase the PCEs of these devices further, it will be necessary to improve the amount of light absorbed by the active polymer. For example, the absorption band of rr-P3HT covers wavelengths only of less than 650 nm, whereas the maximum photon flux of sunlight is located at 700 nm (1.77 eV).11 Thus, it is criti-cal that the band gap of the polymer be narrowed so that it can absorb more photons (relative to rr-P3HT) from sunlight and, thereby, increase the photocurrent and the PCE of the corresponding devices.
Several low-band gap conjugated polymers12–17 have been synthesized that feature a main chain donor/acceptor struc-ture to increase the degree of intrachain electron transfer,
thereby resulting in partial charge separation along the poly-mer backbone and a lower band gap. Moreover, increasing the degree of delocalization of the p-electrons by increasing the coplanarity of the polymeric structure can also lead to a lower band gap.18–21
Carbazole derivatives are electron-donating materials that ex-hibit good hole-transporting characteristics22–24; copolymers based on carbazole derivatives display good performance in solar cell applications.25–30For example, poly[N-90 -hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30 -benzo-thiadiazole)] (PCDTBT) possesses a band gap of 1.88 eV and exhibits a high PCE of 6.1%.31 Conjugated polymers contain-ing imidazole-moieties as acceptor units have also been applied in solar cells exhibiting good performance (PCE ¼ 4.1%).31–34
The planar structure of the imidazole moiety linked covalently to the polymeric side chain extends the effective length of conjugation and lowers the band gap. In this study, we prepared a new planar imidazole unit and incorporated it into the polymeric main chain to extend the effective conjugation length of the system and to increase intramolecular charge transfer. Scheme 1 displays our route toward the synthesis of the thiadiazole/benzoimidazole (TDBI) unit M1. We expected the presence of the TDBI moi-eties in the polymer backbone to lower the band gap and that this polymer would possess a suitable lowest unoccupied
Correspondence to: K.-H. Wei (E-mail: [email protected])
molecular orbital (LUMO) and sufficiently large LUMO offset for efficient electron transfer to PCBM. The LUMO energy level of the polymer must be positioned above that of PCBM by at least 0.3 eV to overcome the exciton binding energy35,36; if the LUMO energy level of the polymer is very close to that of PCBM, there will be a loss of photocurrent.37Since the elec-tron-rich 2,7-carbazole possess good hole transport proper-ties, incorporating 2,7-carbazole units into polymeric main chains provide not only a high degree of conjugation but also good intramolecular charge transfers.38,39Scheme 2 displays the copolymerization of the TDBI (M1) and carbazole (M2) units, performed using a versatile Suzuki cross-coupling reaction.
EXPERIMENTAL
Materials
2,1,3-Benzothiadiazole40 and M227,41 were prepared accord-ing to reported procedures. Dimethylformamide (DMF) was dried over MgSO4; tetrahydrofuran (THF) and diethyl ether
were dried over Na/benzophenone ketyl. All other reagents were used as received from commercial sources, without further purification.
Measurements and Characterization
1H and 13C NMR spectra were recorded using a Varian
Unity-300 NMR spectrometer. Differential scanning calorime-try (DSC) was performed using a Perkin–Elmer Pyris DSC1 instrument operated at a heating rate of 10C min1under N2 purge. Thermogravimetric analysis (TGA) was performed
using a Du Pont TGA 2950 instrument operated at a heating rate of 10 C min1under a N2purge. The number-average
(Mn) and weight-average (Mw) molecular weights were
mea-sured through gel permeation chromatography (GPC) using a Waters chromatography unit interfaced with a Waters 2414 different refractometer. Three 5-lm Waters styragel columns were connected in series in decreasing order of pore size (104, 103, and 102 Å); polystyrene was the standard and THF was the eluant. UV-Vis absorption spectra were mea-sured using an HP Agilent-8453 diode array spectrophotome-ter. Cyclic voltammetry (CV) was performed using a BAS 100 electrochemical analyzer operated at a scan rate of 100 mV s1; the solvent was anhydrous MeCN containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the
supporting electrolyte. A disk glassy carbon electrode coated with a thin film of polymer was used as the working elec-trode; a Pt wire was the counter elecelec-trode; Ag/Agþ(0.01 M AgNO3) was the reference electrode; ferrocene/ferrocenium
(Fc/Fcþ) was used as the internal standard. Topographic images of the copolymer/PCBM films were obtained using a Digital Instruments Nanoscope III atomic force microscope operated in the tapping mode under ambient conditions.
SCHEME 1Reagents and condi-tions: (i) n-octylmagnesium bro-mide, 78C; then rt, overnight; (ii) Et3N, Me3NHCl, p-TsCl, rt, 2 h; (iii) 4-bromophenol, K2CO3, DMF, 110C, overnight; (iv)n-BuLi, 78 C, 1 h; then dry DMF, rt, over-night; (v) Br2, HBr(aq), 100 C, 3 h; (vi) TfOH, HNO3, H2SO4, rt, over-night; (vii) 2-tributylstannylthio-phene, PdCl2(PPh3)2, THF, 80 C, 20 h; (viii) Fe powder, AcOH, 80C, 5 h; (ix)4, H2O2, HCl, DMF, rt, 1 h; (x) NBS, 0C, 1 h.
SCHEME 2Reagents and conditions: (i) tetraethylammonium hydroxide, Pd2dba3, THF, 80C, 8 h.
Solar Cells Device Fabrication and Characterization An indium tin oxide (ITO, 10 X) coated glass substrate was cleaned sequentially with detergent, DI water, acetone, and isopropyl alcohol and then dried in the oven. Before use, the substrate was treated with oxygen plasma. The active layers were prepared by dissolvingPCTDBI and PCBM at different weight ratios but with a fixed total concentration (1.5 wt %) in 1,2-dichlorobenzene, and then the solutions were spin-coating at a speed of 1500 rpm onto the ITO/poly(3,4-ethyl-enedioxthiophene):poly(styrene sulfonate) (PETDOT:PSS) (30 nm) substrates for 60 s (The model of spin coater is Laurell WS-400B-6NPP/LITE). The films were dried at room temper-ature. The thickness of each active layer was 90–100 nm. Finally, Ca (40 nm) was thermally evaporated through a shadow mask and then Al (100 nm) was evaporated to form the top electrode, and the device area of 0.12 cm2. All fabri-cations were performed in nitrogen-filled glovebox. The devi-ces were measured under AM 1.5 radiation (100 mW cm2) using an Agilent 4156 semiconductor parameter analyzer. The spectral mismatch factor was calculated by comparison of the solar simulator spectrum and the AM 1.5 spectrum at room temperature. The external quantum efficiency (EQE) was measured using a system established by Optosolar, Inc. Monochromatic light was created from 500-W Xe lamp source passing through a monochromator. The photocurrent of the device was detected using a lock-in amplifier under short-circuit conditions by illuminating the monochromatic incident beam. A calibrated mono silicon diode exhibiting a response at 300–800 nm was used as a reference.
Synthetic Procedures Heptadecan-9-ol (1)
1-Bromooctane (22.3 mL, 152 mmol) in dry ether (30 mL) was added dropwise to a suspension of Mg (3.24 g, 135 mmol) in dry ether (40 mL) at room temperature. After stir-ring at room temperature for 1 h, the resulting solution was added slowly to a solution of ethyl formate (4.00 g, 54.0 mmol) in dry ether (40 mL) at 78C and then warmed to room temperature. After stirring at room temperature over-night, the mixture was poured into water and extracted with CH2Cl2(3 100 mL). The organic layer was dried (MgSO4)
and the solvent evaporated under reduced pressure. The crude product was purified through recrystallization (MeCN) to obtain1 as a white solid (11.3 g, 82%, mp: 58–62C).
1 H NMR (300 MHz, CDCl3, ppm):d 3.59–3.58 (m, 1H), 1.43– 1.38 (m, 8H), 1.36–1.27 (m, 21H), 0.88 (t, J ¼ 6.9 Hz, 6H). 13 C NMR (75 MHz, CDCl3, ppm): d 72.3, 37.7, 32.1, 30.0, 29.9, 29.5, 25.9, 22.9, 14.3. MS (m/z): [M]þ calcd. for C17H36O, 456.3; found 455. 9-Heptadecane p-Toluenesulfonate (2) A solution of p-TsCl (4.09 g, 25.8 mmol) in CH2Cl2(20 mL)
was added dropwise to a solution of 1 (6.00 g, 23.4 mmol), Et3N (8.14 mL, 58.51 mmol), and Me3NHCl (2.23 g, 24.4
mmol) in CH2Cl2(30 mL) in a 100-mL flask at room
temper-ature. After stirring for 2 h, the mixture was poured into water and extracted with CH2Cl2(3 200 mL). The organic
phase was dried (MgSO4) and the solvent evaporated under
reduced pressure. The crude product was purified through column chromatography (SiO2, EtOAc/hexane 1:9, Rf : 0.76)
to obtain2 as a colorless oil (8.64 g, 90%, mp: 50–54C).
1 H NMR (300 MHz, CDCl3, ppm):d 7.79 (d, J ¼ 8.1 Hz, 2H), 7.32 (d, J ¼ 8.1 Hz, 2H), 4.56–4.52 (m, 1H), 2.44 (s, 3H), 1.56–1.54 (m, 4H), 1.27–1.17 (m, 24H), 0.88 (t, J ¼ 6.9 Hz, 6H). 13C NMR (75 MHz, CDCl3, ppm): d 144.9, 135.1, 129.8, 128.0, 84.9, 34.4, 32.1, 29.6, 29.5, 29.4, 24.9, 22.9, 21.8, 14.3. MS (m/z): [M]þcalcd. for C24H42O3S, 410.3; found 410.
1-Bromo-4-(heptadecan-9-yloxy)benzene (3)
A mixture of 2 (2.00 g, 4.87 mmol), 4-bromophenol (1.01 g, 5.84 mmol), K2CO3 (3.36 g, 24.4 mmol), and DMF (60 mL)
was stirred overnight at 110C in a three-necked flask. After cooling to room temperature, the reaction mixture was poured into water and extracted with EtOAc (3 100 mL). The organic phase was then washed with water and brine. After evaporating the solvent, the crude product was purified through column chromatography (SiO2, hexane, Rf: 0.84) to
obtain3 as a colorless oil (1.79 g, 90%).
1 H NMR (300 MHz, CDCl3, ppm):d 7.34 (d, J ¼ 8.7 Hz, 2H), 7.75 (d, J ¼ 8.7 Hz, 2H), 4.18–4.11 (m, 1H), 1.64–1.61 (m, 4H), 1.40–1.25 (m, 24H), 0.85 (t, J ¼ 7.2 Hz, 6H). 13C NMR (75 MHz, CDCl3, ppm): d, 157.9, 132.2, 117.6, 112.3, 78.5, 33.8, 31.8, 29.7, 29.5, 29.2, 25.3, 22.7, 14.1. MS (m/z): [M]þ
calcd. for C23H39BrO, 410.22; found 410.
4-(Heptadecan-9-yloxy)benzaldehyde (4)
n-BuLi (2.5 M in hexane, 1.17 mL, 2.94 mmol) was added dropwise to a solution of3 (1.00 g, 2.44 mmol) in dry THF (10 mL) in a 50-mL three-necked flask at78C. After stir-ring for 1 h at78C, dry DMF (0.280 mL, 3.62 mmol) was added to the reaction mixture, which was then stirred at room temperature for 8 h. The reaction mixture was poured into water and extracted with EtOAc (3 100 mL). The or-ganic phase was dried (MgSO4) and the solvent evaporated
under reduced pressure. The crude product was purified through column chromatography (SiO2, EtOAc/hexane 1:4,
Rf: 0.87) to obtain4 as a viscous oil (0.61 g, 70%). 1 H NMR (300 MHz, CDCl3, ppm):d 9.84 (s, 1H), 7.79 (d, J ¼ 8.7 Hz, 2H), 6.79 (d, J ¼ 8.7 Hz, 2H), 4.34–4.31 (m, 1H), 1.66–1.60 (m, 4H), 1.40–1.23 (m, 24H), 0.85 (t, J ¼ 7.2 Hz, 6H). 13C NMR (75 MHz, CDCl 3, ppm): d 190.7, 164.0, 132.1, 129.4, 115.6, 78.4, 33.8, 31.8, 29.6, 29.5, 29.2, 25.3, 22.6, 14.1. MS (m/z): [M]þcalcd. for C 24H40O2, 360.3; found 361. 4,7-Dibromo-2,1,3-benzothiadiazole (5)
Br2(3.36 mL, 65.5 mmol) was added slowly to a mixture of
2,1,3-benzothiadiazole (3.00 g, 22.0 mmol) and HBr(aq) (6.6
mL) at 100C, followed by another charge of HBr(aq)(5 mL).
After stirring at 100 C for 3 h, the reaction mixture was cooled to room temperature, poured into a saturated Na2S2O3 solution, and then filtered. The crude product was
purified through recrystallization (EtOH) to obtain 5 as a white solid (5.1 g, 80%, mp: 184–188C).
1
H NMR (300 MHz, CDCl3, ppm): d 7.71 (s, 2H). 13
C NMR (75 MHz, CDCl3, ppm): d 153.3, 132.7, 114.2. MS (m/z):
[M]þcalcd. for C6H2Br2N2S, 293.8; found 293.
4,7-Dibromo-5,6-dinitro-2,1,3-benzothiadiazole (6)
Trifluoromethanesulfonic acid (7.53 mL, 85.0 mmol) and HNO3 (1.8 mL) were added dropwise to H2SO4 (10 mL) at
0C.5 (2.00 g, 6.85 mmol) was added to the acid mixture at 0C; the system kept at room temperature overnight before being poured onto ice-water and then filtered. The filter cake was washed with water and dried. The crude product was purified through column chromatography (SiO2, EtOAc/
hexane 1:9, Rf : 0.34) to obtain 6 as a white solid (1.28 g,
48%, mp: 190–195C). 13 C NMR (75 MHz, CDCl3, ppm): d 151.3, 127.5, 110.3. MS (m/z): [M]þcalcd. for C 6Br2N4O4S, 383.8; found 384. 5,6-Dinitro-4,7-dithien-2-yl-2,1,3-benzothiadiazole (7) PdCl2(PPh3)2(75 mg, 0.11 mmol) was added to a solution of
2-tributylstannylthiophene (3.50 mL, 11.0 mmol) and 6 (1.40 g, 3.67 mmol) in dry THF (20 mL) and then the reac-tion mixture was heated at 80C for 20 h. After cooling to room temperature, the solvent was evaporated and the crude product washed with hexane and dried to yield 7 as an orange solid (1.2 g, 84.2%, mp: 240–245C). 1 H NMR (300 MHz, CDCl3, ppm):d 7.75 (dd, J ¼ 1.2, 5.1 Hz, 2H), 7.52 (dd, J ¼ 1.2, 3.6 Hz, 2H), 7.25–7.22 (m, 2H). 13C NMR (75 MHz, CDCl3, ppm): d 152.2, 141.8, 131.4, 131.0, 129.5, 128.0, 121.5. MS (m/z): [M]þ calcd. for C 14H6N4O4S3, 390.0; found 390. 5,6-Diamino-4,7-dithien-2-yl-2,1,3-benzothiadiazole (8) AcOH (50 mL) was added to a mixture of 7 (1.00 g, 2.56 mmol) and Fe powder (1.70 g, 30.4 mmol). The mixture was heated at 80C for 5 h before being cooled to room temper-ature, poured into the water, and extracted with ether (3 100 mL). The organic phase was washed sequentially with 5% NaOH(aq)and water and then dried (MgSO4). Evaporation
of the solvent under reduced pressure yielded 8 as an orange solid (0.77 g, 91%, mp: 219–225C). 1 H NMR (300 MHz, CDCl3, ppm):d 7.56 (dd, J ¼ 1.2, 5.1 Hz, 2H), 7.36 (dd,J ¼ 1.2, 3.6 Hz, 2H), 7.25 (t, J ¼ 3.6 Hz, 2H), 4.39 (s, 4H).13C NMR (75 MHz, CDCl3, ppm):d 151.2, 139.7, 135.6, 128.8, 127.8, 127.5, 107.4. MS (m/z): [M]þ calcd. for C14H10N4S3, 330.0; found 330. 4,10-Bis(thiophene-2-yl)-7-[4-(heptadecan-9-yloxy)-phenyl]-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole (9) H2O2(0.87 mL) and HCl (0.34 mL) were added to a solution
of8 (0.400 g, 1.21 mmol) and 4 (0.52 g, 1.45 mmol) in DMF
(15 mL). After stirring at room temperature for 1 h, the reaction mixture was poured into water and extracted with EtOAc (3 100 mL). The organic layer was dried (MgSO4)
and the solvent evaporated under reduced pressure. The crude product was purified through column chromatography (SiO2, EtOAc/hexane 1:9, Rf: 0.58) to yield 9 as a red solid
(0.41 g, 50%, mp: 140–145C). 1 H NMR (300 MHz, CDCl3, ppm): d 9.63 (s, 1H), 9.07 (dd, J ¼ 1.2, 3.9 Hz, 1H), 8.12 (d, J ¼ 8.7 Hz, 2H), 8.01 (dd, J ¼ 1.2, 3.9 Hz, 1H), 7.56 (td,J ¼ 0.9, 5.7 Hz, 2H), 7.35–7.29 (m, 2H), 7.04 (d, J ¼ 8.7 Hz, 2H), 4.37–4.33 (m, 1H), 1.69-1.55 (m, 4H), 1.50–1.24 (m, 24H), 0.86 (t, J ¼ 7.2 Hz, 6H). 13C NMR (75 MHz, CDCl3, ppm): d 162.3, 156.8, 150.6, 150.1, 144.6, 137.1, 136.8, 136.6, 130.9, 128.9, 128.3, 128.1, 127.3, 125.9, 120.2, 114.8, 113.8, 104.4, 98.8, 78.2, 33.8, 31.8, 29.7, 29.5, 29.3, 25.4, 22.6, 14.1. MS (m/z): [M]þ calcd. for C38H46N4OS3, 670.3; found 671.
TABLE 1Molecular Weights and Thermal Properties ofPCTDBI Mn Mw PDI Tg(C) Td(C)a
PCTDBI 25,600 32,560 1.27 105 396
a
Temperature at which 5% loss of the initial weight occurred.
FIGURE 1 (a) Normalized UV-Vis absorption spectra ofPCTDBI andM1. (The concentrations of solution were 1 105M; film (93 nm) was spin-coated on quartz glass). (b) Absoprtion coeffi-cient of PCTDBI film (90 nm). The film was spin-coated on quartz glass).
4,10-Bis(5-bromothiophene-2-yl)-7-[4-(heptadecan-9-yloxy)-phenyl]-6H-[1,2,5] thiadiazole[3,4-g]benzoimidazole (M1)
NBS (0.270 g, 1.49 mmol) was added in five portions to a solution of9 (0.500 g, 0.910 mmol) in DMF (15 mL) at 0C. After 1 h, the mixture was poured into water and extracted with EtOAc (3 100 mL). The organic phase was dried (MgSO4) and the solvent evaporated under reduced pressure.
The crude product was purified through column chromato-graphy (SiO2, EtOAc/hexane 1:9, Rf : 0.74) to yield M1 as
a red solid (0.35 g, 57%, mp: 95–100C). 1 H NMR (300 MHz, CDCl3, ppm):d 9.27 (s, 1H), 8.73 (d, J ¼ 4.2 Hz, 1H), 7.92 (d, J ¼ 8.7 Hz, 2H), 7.61 (d, J ¼ 3.9 Hz, 1H), 7.26–7.25 (m, 1H), 7.19 (d,J ¼ 3.9 Hz, 1H), 6.95 (d, J ¼ 8.7 Hz, 2H), 4.38–4.34 (m, 1H), 1.68–1.58 (m, 4H), 1.46–1.25 (m, 24H), 0.86 (t,J ¼ 7.2 Hz, 6H). 13C NMR (75 MHz, CDCl3, ppm): d 162.6, 156.9, 151.6, 149.1, 138.5, 138.3, 133.2, 133.1, 131.6, 130.2, 129.0, 128.7, 119.8, 119.4, 117.2, 115.5, 114.9, 114.7 110.0, 78.3, 33.8, 31.9, 29.8, 29.5, 29.3, 25.3, 22.6, 14.1. Anal. Calcd. for C38H44Br2N4OS3: C, 55.07; H, 5.35;
N, 6.76. Found: C, 55.35; H, 5.62; N, 6.80%. MS (m/z): [M]þ calcd. For C38H44Br2N4OS3, 828.8; found 829.
PCTDBI
A mixture ofM1 (100 mg, 0.120 mmol), M2 (79.6 mg, 0.120 mmol), and tetraethylammonium hydroxide (0.46 mL) in THF (1.60 mL) was degassed at 50C for 10 min. Tri(diben-zlideneacetone)dipalladium(0) (2.20 mg, 2.40 lmol) and tri-phenylphosphate (2.50 mg, 9.60lmol) were added and then the reaction mixture was heated under reflux at 80 C for
8 h. Phenylboronic acid (40.8 mg, 0.250 mmol) was then added and the mixture stirred for 1 h at 80 C. Bromoben-zene (30.0lL, 0.250 mmol) was then added and the mixture stirred for another 1 h at 80 C. The resulting mixture was poured into MeOH (50 mL) and filtered. The precipitated material was washed with acetone for 72 h in a Soxhlet apparatus and then dried (75 mg, 58%).
1
H NMR (300 MHz, CDCl3, ppm): d 9.04 (br, 2H), 7.97 (br,
4H), 7.59 (br, 5H), 7.05 (br, 2H), 6.63 (br, 1H), 4.71 (br, 1H), 4.34 (br, 1H), 2.37 (br, 2H), 1.66 (br, 8H), 1.32–1.15 (m,
FIGURE 2CV trace ofPCTDBI.
FIGURE 3Current density-voltage characteristics of illuminated (AM 1.5G, 100 mW cm2) polymer solar cells incorporating active layers of (a)PCTDBI/PCBM and (b) PCTDBI/PC71BM. TABLE 2Optical and Electrochemical Properties ofPCTDBI
kmax(nm) sol kmax(nm) film Eoptg (eV)a Eonsetox (V) Eredonset(V) HOMO (eV) LUMO (eV) Eecg (eV)b
PCTDBI 394,596 408,620 1.75 0.33 1.83 5.13 2.97 2.16
a
Calculated from the edge of the absorption spectrum of the film. bEec
44H), 0.87–0.77 (m, 12H). Anal. Calcd. for C67H85N5OS3: C,
75.02; H, 7.99; N, 6.53. Found: C, 72.44; H, 7.87; N, 6.09%.
RESULTS AND DISCUSSION
Synthesis and Characterization
Schemes 1 and 2 display the synthetic routes that we used to prepare the monomer M1 and the copolymer PCTDBI.
Heptadecan-9-ol (1), synthesized from n-octylmagnesium bromide and ethyl formate, was reacted with p-TsCl to yield 9-heptadecanep-toluenesulfonate (2), which we then treated with 4-bromophenol to obtain 1-bromo-4-(heptadecan-9-yloxy)benzene (3). 4-(Heptadecan-9-yloxy)benzaldehyde (4) was synthesized through carbonylation of compound 3. 4,7-Dibromo-5,6-dinitro-2,1,3-benzothiadiazole (6) was obtained through bromination and nitration of 2,1,3-benzothiadiazole; it was then treated with 2-tributylstannylthiophene to yield 5,6-dinitro-4,7-dithien-2-yl-2,1,3-benzothiadiazole (7) via Stille coupling; reduction of the nitro groups provided 5,6-diamino-4,7-dithien-2-yl-2,1,3-benzothiadiazole (8). We obtained 5,6-diamino-4,7-dithien-2-yl-2,1,3-benzothiadiazole (9) from the condensation of 4 and 8. Subsequently, we pre-pared M1 through the bromination of 9. We synthesized PCTDBI through Suzuki cross-coupling copolymerization of M1 and M2. The structures of the monomers and polymer were confirmed using1H and13C NMR spectroscopy and ele-mental analysis. Table 1 lists the molecular weights, thermal decomposition temperature, and glass transition temperature of the polymer. The number-average weight (Mn) and
poly-dispersity index (PDI) of polymer were 25,600 g mol1and 1.27, respectively. This polymer exhibited good solubility in 1,2-dichlorobenzene and good thermal stability, as evidenced
FIGURE 4Topographic images ofPCTDBI/PCBM films deposited at blend ratios of (a) 1:1, (b) 1:2, (c) 1:3, and (d) 1:4. TABLE 3Photovoltaic of Bulk Heterojunction Solar Cellsa
Weight Ratio of Active Layer Voc (V) Jsc (mA cm2) FF PCE (%) PCTDBI/PCBM ¼ 1:1 0.69 4.57 0.28 0.88 PCTDBI/PCBM ¼ 1:2 0.64 5.89 0.32 1.20 PCTDBI/PCBM ¼ 1:3 0.67 4.18 0.31 0.86 PCTDBI/PCBM ¼ 1:4 0.64 2.69 0.34 0.58 PCTDBI/PC71BM¼ 1:1 0.74 5.52 0.33 1.35 PCTDBI/PC71BM¼ 1:2 0.75 6.46 0.38 1.84 PCTDBI/PC71BM¼ 1:3 0.70 5.27 0.30 1.10 PCTDBI/PC71BM¼ 1:4 0.72 4.37 0.29 0.91 a
by its 5% weight-loss temperature (Td) of 396 C and glass
transition temperature (Tg) of 105C.
Optical Properties
Figure 1(a) presents the absorption spectra of the polymer in dilute CHCl3 solution (1 105 M) and as a solid film
(93 nm) on quartz glass; Table 2 summarizes the corre-sponding optical data. In solution,PCTDBI exhibits two main absorption bands at 320–480 and 490–720 nm, respectively. Relative to the absorption spectrum of TDBI unit (M1), the absorption peak of PCTDBI exhibited a red shift of 78 nm that was caused by an increase in the conjugation length and the presence of intramolecular charge transport between TDBI and carbazole units. The longer-wavelength absorption maximum of PCTDBI was located at 596 nm; this signal appeared at a higher wavelength than that of the corre-sponding carbazole-derivative polymer featuring benzothia-diazole segments as the acceptor (545 nm),27 presumably because the presence of the coplanar imidazole structure in the polymer backbone increased the degree of coplanarity which provided a longer effective conjugation length and better intramolecular charge transfer.42,43 The absorption spectrum of the polymer thin film was red-shifted related to its solution spectrum, suggesting intermolecular interactions and aggregation in the solid state. Figure 1(b) displays the absorption coefficient (1.09 105 at kmax ¼ 620 nm) of
PCTDBI film (90 nm) on quartz glass. The optical band gap (1.75 eV), calculated from the absorption edge (705 nm) of the solid state film, is very close to the photon flux maxi-mum of the solar spectrum (1.77 eV), suggesting that PCTDBI might be a useful material for solar cell applications.
Electrochemical Properties
Cyclic voltammetry is used widely to investigate the redox behavior of polymers and to estimate their highest occupied molecular orbital (HOMO) and LUMO energy levels. We recorded the CV traces ofPCTDBI in MeCN containing 0.1 M TBAPF6at a potential scan rate of 100 mV s1. Figure 2
dis-plays the electrochemical behavior of PCTDBI; Table 2 sum-marizes the CV data. PCTDBI exhibited reversible oxidation and reduction processes, the oxidation and reduction onset was located at 0.33 V and 1.83 V that were attributed to the oxidation and reduction capability of PCTDBI, respec-tively. The HOMO and LUMO energy levels of PCTDBI were 5.13 and 2.97 eV, respectively, according to the energy level of the ferrocene reference (4.8 eV below vacuum level).44,45 The higher HOMO level of PCTDBI than that of other carbazole-containing polymers, is presumably caused by the presence of electron-donating alkoxy groups in PCTDBI’s side chain,46,47and the higher
PCTDBI’s HOMO in turn results in a lowerVocfor its heterojunction device. The
LUMO offset was 1.13 eV relative to the LUMO energy level of PCBM (4.1 eV),48 suggesting that electrons could be transported efficiently to PCBM. The electrochemical band gap (Eec
g) was higher than the optical band gap (Eoptg ),
pre-sumably because of the interface energy barrier between the polymer film and the electrode surface.15,49
Photovoltaic Properties
Figure 3 displays the photovoltaic properties of devices hav-ing the structure ITO/PEDOT:PSS/PCTDBI:PCBM/Ca/Al under AM 1.5G illumination (100 mW cm2). Table 3 sum-marizes the performance of the devices. These devices exhib-ited open circuit voltages (Voc) of 0.64–0.69 V, which were
related to the energy difference between the HOMO energy level of the copolymer and the LUMO energy level of PCBM.50,51When we increased the amount of PCBM from a blend ratio of 1:1 to 1:2, the short circuit current density (Jsc) increased significantly; it decreased upon increasing the
loading weight ratio of PCBM to 1:3 and 1:4 because of the unfavorable morphologies of the active layer. Figure 4 dis-plays AFM images of the surface morphologies of blended films of PCTDBI/PCBM, prepared from 1,2-dichlorobenzene solutions using a procedure identical to that used to fabri-cate the active layers of the devices. For PCTDBI/PCBM at
FIGURE 5(a) Absorption spectra of active layer at various PCTDBI/PCBM and PCTDBI/PC71BM blend ratios. (b) Spectra of EQE spectra of active layers based on PCTDBI/PCBM and PCTDBI/PC71BM (blend ratio, 1:2).
weight ratios were 1:3 and 1:4, the images reveal rougher surfaces compared with that obtained at a blend ratio at 1:2. These rougher surfaces presumably arose from poor misci-bility betweenPCTDBI and PCBM, which leaded to aggrega-tion of PCBM that might limit the degree of charge dissocia-tion and reduced the photocurrent.52–54 The AFM images suggested that devices based on high PCTDBI/PCBM weight ratios (1:3 or 1:4) would exhibit inefficient electron trans-port to the electrode and, thus, lower values of Jsc.
Accord-ingly, we obtained the highest value of Jsc (5.89 mA cm2)
and PCE (1.20%) at a blend ratio of 1:2. We also fabricated a solar cell device incorporating PC71BM in place of PCBM as
the electron acceptor. Figure 5(a) reveals the absorption spectra of the active layer at variousPCTDBI with PCBM or PC71BM weight ratios. As compare with the absorption
spec-tra ofPCTDBI/PCBM, the spectra of PCTDBI/PC71BM exhibit
a broad absorption peaks around 500 nm that was contrib-uted from PC71BM. Figure 5(b) displays the external
quan-tum efficiency (EQE) curves of the two devices; we observe higher quantum efficiency at wavelengths below 550 nm for the PCTDBI/PC71BM device, which was presumably caused
by the absorption of PC71BM in the shorter-wavelength
range and thus increased photocurrent.3 The short circuit
currents decreased when we increased the blend weight ratios of PCTDBI/PC71BM to 1:3 and 1:4, similar to the
trend we observed forPCTDBI/PCBM. The device based on PCTDBI/PC71BM (1:2) exhibited the highest values of Jsc
(6.46 mA cm2) and PCE (1.84%).
CONCLUSIONS
The polymerPCTDBI, prepared through Suzuki coupling and containing alternating carbazole and TDBI units in its main chain, possesses good thermal properties (Tg¼ 105C;Td¼
396 C) and decent number-average molecular weight (25,600 g mol1). The band gap of this polymer was 1.75 eV as a result of incorporating planar thiadiazole/benzoimida-zole units into the polymeric backbone. Moreover, this poly-mer exhibited a suitable LUMO energy level to allow efficient charge dissociation. A solar cell device fabricated with PCTDBI/PCBM at a blend ratio of 1:2 (w/w) exhibited a value ofJscof 5.89 mA cm2and a PCE of 1.20%; these
val-ues increased to 6.46 mA cm2and 1.84%, respectively, for the corresponding device incorporating PCTDBI/PC71BM at
a blend ratio of 1:2 (w/w).
The authors thank the National Science Council, Taiwan, for financial support (NSC 98-2120M-009-006).
REFERENCES AND NOTES
1 Egbe, D. A. M.; Nguyen, L. H.; Schmidtke, K.; Wild, A.; Sieber, C.; Guenes, S.; Sariciftci, N. S. J Polym Sci Part A: Poly Chem 2007, 45, 1619–1631.
2 Zhang, S.; Guo, Y.; Fan, H.; Liu, Y.; Chen, H. Y.; Yang, G.; Zhan, X.; Liu, Y.; Li, Y.; Yang, Y. J Polym Sci Part A: Polym Chem 2009, 47, 5498–5508.
3 Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hum-melen, J. C.; van Hal, P. A.; Janssen, R. A. J Angew Chem Int Ed 2003, 42, 3371–3375.
4 Zhang, F.; Svensson, M.; Andersson, M. R.; Maggini, M.; Bucella, S.; Menna, E.; Ingana¨s, O. Adv Mater 2001, 13, 1871–1874.
5 Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Sariciftci, N. S. Adv Funct Mater 2004, 14, 1005–1011.
6 Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222–225.
7 Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat Mater 2005, 4, 864–868.
8 Chiu, M. Y.; Jeng, U. S.; Su, C. H.; Liang, K. S.; Wei, K. H. Adv Mater 2008, 20, 2573–2578.
9 Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stu¨hn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. J. Adv Funct Mater 2005, 15, 1193–1196.
10 Kim, K.; Liu, J.; Namboothiry, M. A. G.; Carroll, D. L. Appl Phys Lett 2007, 90, 163511.
11 Coakley, K. M.; McGehee, M. D. Chem Mater 2004, 16, 4533–4542.
12 Beaujuge, P. M.; Subbiah, J.; Choudhury, K. R.; Ellinger, S.; McCarley, T. D.; So, F.; Reynolds, J. R. Chem Mater 2010, 22, 2093–2106.
13 Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv Mater 2008, 20, 2556–2560.
14 Huo, L.; Tan, Z. A.; Wang, X.; Zhou, Y.; Han, M.; Li, Y. J Polym Sci Part A: Polym Chem 2008, 46, 4038–4049.
15 Wang, E.; Wang, M.; Wang, L.; Duan, C.; Zhang, J.; Cai, W.; He, C.; Wu, H.; Cao, Y. Macromolecules 2009, 42, 4410–4415. 16 Helgesen, M.; Gevorgyan, S. A.; Krebs, F. C.; Janssen, R. A. J. Chem Mater 2009, 21, 4669–4675.
17 Mishra, S. P.; Palai, A. K.; Srivastava, R.; Kamalasanan, M. N.; Patri, M. J Polym Sci Part A: Polym Chem 2009, 47, 6514–6525.
18 Chen, G. Y.; Chiang, C. M.; Kekuda, D.; Lan, S. C.; Chu, C. W.; Wei, K. H. J Polym Sci Part A: Polym Chem 2010, 48, 1669–1675.
19 Becerril, H. A.; Miyaki, N.; Tang, M. L.; Mondal, R.; Sun, Y. S.; Mayer, A. C.; Parmer, J. E.; McGehee, M. D.; Bao, Z. J Mater Chem 2009, 19, 591–593.
20 Mondal, R.; Miyaki, N.; Becerril, H. A.; Norton, J. E.; Parmer, J.; Mayer, A. C.; Tang, M. L.; Bre´das, J. L.; McGehee, M. D.; Bao, Z. Chem Mater 2009, 21, 3618–3628.
21 Li, K. C.; Hsu, Y. C.; Lin, J. T.; Yang, C. C.; Wei, K. H.; Lin, H. C. J Polym Sci Part A: Polym Chem 2009, 47, 2073–2092. 22 Xie, L. H.; Deng, X. Y.; Chen, L.; Chen, S. F.; Liu, R. R.; Hou, X. Y.; Wong, K. Y.; Ling, Q. D.; Huang, W. J Polym Sci Part A: Polym Chem 2009, 47, 5221–5229.
23 Morin, J. F.; Drolet, N.; Tao, Y.; Leclerc, M. Chem Mater 2004, 16, 4619–4626.
24 Kobayashi, N.; Koguchi, R.; Kijima, M. Macromolecules 2006, 39, 9102–9111.
25 Zhou, E.; Cong, J.; Yamakawa, S.; Wei, Q.; Nakamura, M.; Tajima, K.; Yang, C.; Hashimoto, K. Macromolecules 2010, 43, 2873–2879.
26 Zou, Y.; Gendron, D.; Aı¨ch, R. B.; Najari, A.; Tao, Y.; Leclerc, M. Macromolecules 2009, 42, 2891–2894.
27 Blouin, N.; Michaud, A.; Leclerc, M. Adv Mater 2007, 19, 2295–2300.
28 Qin, R.; Li, W.; Li, C.; Du, C.; Veit, C.; Schleiermacher, H. F.; Andersson, M.; Bo, Z.; Liu, Z.; Ingana¨s, O.; Wuerfel, U.; Zhang, F. J Am Chem Soc 2009, 131, 14612–14613.
29 Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Plesu, R. N.; Belleteˆte, M.; Durocher, G.; Tao, Y.; Leclerc, M. J Am Chem Soc 2008, 130, 732–742.
30 Yuan, M. C.; Su, M. S.; Chiu, M. Y.; Wei, K. H. J Polym Sci Part A: Polym Chem 2010, 48, 1298–1309.
31 Park, S. H.; Roy, A.; Beaupre´, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Pho-tonics 2009, 3, 297–303.
32 Chang, Y. T.; Hsu, S. L.; Su, M. H.; Wei, K. H. Adv Funct Mater 2007, 17, 3326–3331.
33 Chang, Y. T.; Hsu, S. L.; Chen, G. Y.; Su, M. H.; Sing, T. A.; Diau, E. W. G.; Wei, K. H. Adv Funct Mater 2008, 18, 2356–2365. 34 Chang, Y. T.; Hsu, S. L.; Su, M. H.; Wei, K. H. Adv Mater 2009, 21, 2093–2097.
35 Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl Phys Lett 2006, 88, 093511.
36 Brabec, C. J.; Winder, C.; Sariciftci, N. S.; Hummelen, J. C.; Dhanabalan, A.; van Hal, P. A.; Janssen, R. A. J Adv Funct Mater 2002, 12, 709–712.
37 Zhang, F.; Bijleveld, J.; Perzon, E.; Tvingstedt, K.; Barrau, S.; Ingana¨s, O.; Andersson, M. R. J Mater Chem 2008, 18, 5468–5474.
38 Morin, J. F.; Leclerc, M.; Ade`s, D.; Siove, A. Macromol Rapid Commun 2005, 26, 761–778.
39 Leclerc, N.; Michaud, A.; Sirois, K.; Morin, J. F.; Leclerc, M. Adv Funct Mater 2006, 16, 1694–1704.
40 Neto, B. A. D.; Lopes, A. S. A.; Ebeling, G.; Goncalves, R. S.; Costa, V. E. U.; Quina, F. H.; Dupont, J. Tetrahedron 2005, 61, 10975–10982.
41 Freeman, A. W.; Urvoy, M.; Criswell, M. E. J Org Chem 2005, 70, 5014–5019.
42 Li, Y.; Li, H.; Xu, B.; Li, Z.; Chen, F.; Feng, D.; Zhang, J.; Tian, W. Polymer 2010, 51, 1786–1795.
43 Li, Y.; Li, Z.; Wang, C.; Li, H.; Lu, H.; Xu, B.; Tian, W. J Polm Sci Part A: Polm Chem 2010, 48, 2765–2776.
44 Zhou, E.; Yamakawa, S.; Tajima, K.; Yang, C.; Hashimoto, K. Chem Mater 2009, 21, 4055–4061.
45 Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Ba¨ssler, H.; Porsch, M.; Daub, J. Adv Mater 1995, 7, 551–554. 46 Chen, H. Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Nat Photonics 2009, 3, 649–653. 47 Shi, C.; Yao, Y.; Yang, Y.; Pei, Q. J Am Chem Soc 2006, 128, 8980–8986.
48 Mu¨hlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. Adv Mater 2006, 18, 2884–2889. 49 Egbe, D. A. M.; Nguyen, L. H.; Hoppe, H.; Mu¨hlbacher, D.; Saricific, N. S. Macromol Rapid Commun 2005, 26, 1389–1394. 50 Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, S.; From-herz, T.; Rispens, M. T.; Sanchez, L.; Hummelem, J. C. Adv Funct Mater 2001, 11, 374–380.
51 Scharber, M. C.; Mu¨hlbacher, D.; Koppe, M.; Denk, P.; Wal-dauf, C.; Heeger, A. J.; Brabec, C. J Adv Mater 2006, 18, 789–794.
52 Huang, J. H.; Ho, Z. Y.; Kekuda, D.; Chang, Y.; Chu, C. W.; Ho, K. C. Nanotechnology 2009, 20, 025202.
53 Hoppe, H.; Sariciftci, N. S. J Mater Chem 2006, 16, 45–61. 54 Nilsson, S.; Bernasik, A.; Budkowski, A.; Moons, E. Macro-molecules 2007, 40, 8291–8301.
